Take a look at this SilverZine tutorial posted by Phil Middlemiss and find out how to animate the Silverlight opacity mask.
In the context of animation, one of the biggest features in Flash that is missing from Silverlight 3 is an Opacity Mask layer. I haven't seen any mention of this in to Silverlight 4, but I could be mistaken. The only feature that we have in Silverlight 3 is the OpacityMask on each element, which is either a Brush or an image - (Image Brushes don't tile either, but that's a post for another day).